"The metropolitan coast is quite exposed with high levels of water and sand movement so traditional methods of transplanting seagrass seedlings don't work because they simply get washed away," he said.

"The biodegradable hessian sandbags allow more seagrass seeds to take root and grow."

The sandbags are being dropped from the SARDI research boat the Ngerin across two areas of the seabed and the divers will check in six months to see whether there is new growth.

In areas where the hessian bags have been spread previously, thriving seagrass has been found more than five years later.

Marine experts say seagrass meadows are the feeding grounds for many South Australian marine species, including King George whiting and garfish.

It is estimated more than 5,200 hectares of seagrass have been lost along the Adelaide coast in the past half century.

Factors blamed include wastewater and stormwater run-off and more industry and other coastal development.